that I object to from so-called "technicians" and "shops".
"Positive pressure breathing", which is what Mike is referring to, is not a good thing. There is also no reason for it, other than to skew regulator testing results (fortunately most testers now makes it clear if this is happening in their tests so you can shake your fist at the offender(s)) It increases gas consumption, possibly radically so (especially if it freeflows on you) and can be dangerous, as you experienced.
The IDEAL situation would be a regulator that has ZERO resistance to airflow - that is, it would mimic free air exchange with your lungs and the outside ambient air. It would require neither that you "suck" nor would it blow air down your throat.
Of course what you have outside of your mouth underwater isn't air - its water. But the principle is the same.
Its equally undesirable to have a high breathing resistance, because your diaphram has to do WORK to pump the air in and out. The more resistance, the more work. That's bad as well, and can contribute to CO2 buildup.
Positive pressure breathing is an attempt by regulator makers to cause a regulator to have a POSITIVE feedback influence when it begins to flow air, as a consequence of the flow in the regulator body itself. That is, air flows, that air "sucks" the diaphram inward due to a venturi effect (a venturi drops pressure in the chamber), which pulls the lever down further.... the result is a freeflow. The good news is that many regs have this setting adjustable by the diver, and virtually all can be adjusted internally if its not externally accessible.
I strongly disagree that tuning for positive-pressure breathing is a desirable thing, because if you induce a purge underwater, you get what you experienced - a freeflow!
This also means that if you make a sudden kick or turn, and the flow of water presses on the diaphram, you ALSO get a freeflow that won't stop, because the feedback loop gets going and now you're groping around trying to stick your hand over the mouthpiece to stop it.
Mike ALSO referred to something COMPLETELY different (and why makes no sense to me) which is the situation where the housing is pointing upward (mouthpiece up) and is submerged. This WILL produce a freeflow but for a different reason - the cavity behind the diaphram is full of AIR at ambient (its connected to the surface air by the mouthpiece), while the outside under the diaphram has water pressing on it at greater pressure. This SHOULD produce a "flow", but its not a "freeflow" in that as soon as the entire body goes underwater and the pressures equalize on both sides of the diaphram it should STOP.
The latter is normal.
The former indicates that the venturi adjustment is incorrect.
If you want to set your regs up for positive-pressure breathing be my guest, but its not a good idea to do it because then ANY initiation of air flow through them, for any reason, if the reg is not in your mouth to provide backpressure leads to EXACTLY what you experienced.
A REG SHOULD BE STABLE UNDERWATER WITH NO BACKPRESSURE IN THE FORM OF BEING IN YOUR MOUTH! Positive feedback loops are IMHO BAD!
A better place to tune them, IMHO, is JUST SHORT of where a freeflow will sustain itself. This, along with a proper cracking adjustment, produces a regulator that is as close as you can get to "zero inhalation resistance" without crossing the line into positive-pressure breathing. How close you can get with a particular reg without it becoming unstable depends on both its design and condition. I can get a closer to "zero resistance" with my S600, for example, than the R380 - but both breathe just fine underwater and NEITHER is unstable. (Yes, I can tell the difference - but not by much!)
